A significant percentage of people in the US suffer from behavioral disorders, including posttraumatic stress disorder (PTSD), anxiety disorders, and drug addiction, that are effectively diseases of memory dysfunction. Therefore, an understanding of the cell biology of learning and memory should facilitate the development of treatments for these disorders. One of the fundamental forms of associative learning is classical conditioning. During classical conditioning an animal learns to associate a neutral stimulus (the conditioned stimulus) with the delivery of a reinforcing stimulus (unconditioned stimulus). This project will exploit the significant experimental advantages of the marine mollusk Aplysia californica to develop a mechanistic understanding of classical conditioning. This animal has a simple nervous system that is highly suited to electrophysiological, pharmacological, and molecular experimental techniques. The project will focus on a classical conditioning of the defensive withdrawal reflex in Aplysia. This form of learning is mediated by a combination of homosynaptic, NMDA receptor-dependent plasticity and heterosynaptic, serotonergic neuromodulation. The PI hypothesizes that the postsynaptic motor neuron is a major cite of associative interaction between these two processes. The project will investigate the potential postsynaptic interactions between Hebbian and neuromodulatory processes. Toward this end, the PI will perform imaging of calcium in the motor neuron to determine whether intracellular calcium resulting from postsynaptic NMDA receptor activity and calcium released from postsynaptic intracellular stores by serotonin stimulation interact. In addition, the potential rol of postsynaptic calcium- calmodulin kinase II and postsynaptic metabotropic glutamate receptors will be examined. The PI will also investigate the role of modulation of postsynaptic AMPA receptor trafficking in classical conditioning by expressing constructs in Aplysia motor neurons that block AMPA receptor trafficking. The proposed studies will use both sensorimotor cocultures and reduced preparations of Aplysia.